JPWO2020204059A1 - Fracture prediction method for steel materials, fracture prediction device, program and recording medium - Google Patents

Abstract

Enter the plate thickness of the steel material and the limit VDA bending angle of the steel material in the first relational expression defined using the finite element method with the solid element, and the first value which is the maximum principal strain of the outermost surface layer of bending by the solid element. In the first step S11 for calculating the above and the second relational expression defined by using the finite element method using the shell element, the element size of the shell element of the deformation analysis model, the plate thickness of the steel material, and the first value are input. Then, using the second step S12 for calculating the second value, which is the maximum principal strain of the outermost surface layer of bending due to the shell element, and the finite element method using the shell element, the first primary strain, which is the maximum principal strain of the outermost surface layer of bending of the steel material. There is a third step S2 for acquiring the three values, and a fourth step S3 for comparing the second value and the third value and determining whether or not fracture occurs at the bending deformation portion of the steel material in the deformation. Breakage prediction method.

Description

The present invention relates to a fracture prediction method for steel materials, a fracture prediction device, a program, and a recording medium.

In recent years, in the automobile industry, the development of a vehicle body structure capable of reducing the impact at the time of a collision and increasing the impact absorption energy at the time of a collision has become an issue of the past. For example, in the development of such a vehicle body structure, the method of predicting the fracture of a steel sheet, which is used in the field of press forming simulation of a steel sheet, is often applied when predicting the deformation and fracture of the vehicle body structure at the time of a collision. .. As this fracture prediction method, a limit strain called a forming limit diagram is experimentally or theoretically derived, and the limit strain state is compared with the strain state obtained by simulation using the finite element method (FEM). Then, a method of determining the presence or absence of breakage is common.

On the other hand, in recent years, as a method for more accurately evaluating the bending characteristics of steel plates used for automobile bodies at the time of collision, a plate bending test of a metal material according to the test standard (Non-Patent Document 1) of the VDA German Association of the Automotive Industry (Non-Patent Document 1). The method using the VDA bending test) is becoming widespread. In the VDA bending test, a steel sheet is bent while pushing a punch between a pair of rolls, and the bending angle of the steel sheet at the timing when the reaction force of the punch becomes the maximum value is defined as the limit VDA bending angle of the steel sheet. According to this evaluation method, not only the limit bending angle can be determined from the continuous bending angle data, but also there is an advantage that variation by the measurer does not occur.

Japanese Unexamined Patent Publication No. 2012-11458 Japanese Patent No. 6330981 Japanese Unexamined Patent Publication No. 2011-141237

VDA 238-100 "Plate bending test for metallic materials" Validation Rule, 01 June 2017

In Patent Document 1, for the metal plate to be analyzed, the limit surface strain at which cracks occur is obtained in advance by a test, or the limit bending radius at which cracks occur is obtained in advance by a bending test. A technique is disclosed in which the maximum principal strain is obtained as the limit surface strain by a simulation imitating a bending test, a press forming simulation is performed, and the occurrence of cracks is determined by comparison.

When the crack is determined for the metal plate to be analyzed in Patent Document 1, it is necessary to carry out a bending test and a simulation imitating the bending test each time, and the limit surface strain can be obtained for the first time. .. Further, a 90 ° V-bending test by the V-block method is used for the bending test. In the V-block method, a plurality of punches having different bending radii are prepared, and bending is performed with the punches having each bending radius. Determine if cracking occurs with a punch with a bending radius. Therefore, only the data corresponding to the number of punches produced can be obtained, and only fragmentary results can be obtained. Further, in the determination of the crack of the steel sheet, whether or not a minute crack occurs is a determination criterion, so that the determination by the measurer may vary.

In addition, when the limit surface strain is obtained by a simulation that imitates a bending test, the strain is averaged within the element at a site where the strain is locally concentrated, such as the bending top, so the element size is large. The coarser the strain, the smaller the value. Therefore, there is a problem that the calculated strain value differs depending on the element size used.

The present invention has been made in view of the above problems, and the maximum principal strain of the bent outer surface layer of the steel material to be analyzed can be obtained extremely easily, in a short time, and accurately without using tests or simulations. This makes it possible to accurately predict whether or not bending fracture will occur in the projecting deformation portion, that is, to accurately predict bending fracture of the steel material, resulting in a large cost required for product development. It is an object of the present invention to provide a fracture prediction method, a fracture prediction device, a program and a recording medium that can contribute to a significant reduction and a significant reduction in the development period.

In order to solve the above problems, as a result of diligent studies, we came up with the following aspects of the invention. The gist of the present invention is as follows.

(1)
The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. The first step of inputting the limit VDA bending angle of the steel material and calculating the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated in the first step, and enter the outermost bending of the steel material by the shell element. The second step to calculate the second value, which is the maximum principal strain of the surface layer, and
Using the finite element method using the shell element, the third step of performing deformation analysis according to the material properties of the steel material and acquiring the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
The fourth step of comparing the second value with the third value to determine whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
A fracture prediction method characterized by being equipped with.

(2)
In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction method according to (1), wherein a: the relational expression of the plate thickness t and b: the relational expression of the plate thickness t.

(3)
In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture prediction method according to (2), which is represented by a relational expression of shell element size α, β, γ: ε _{solid and plate thickness t.}

(4)
The solid element used to obtain the first relational expression is characterized in that the element size is finer than that of the shell element used to obtain the second relational expression (1) to (3). ) Is described in any of the above methods.

(5)
The fracture prediction method according to any one of (1) to (4), wherein the steel material is a steel sheet of a steel grade of 980 MPa class or higher.

(6)
The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. A first calculation unit that inputs the limit VDA bending angle of the steel material and calculates the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated by the first calculation unit into the equation, and bend the outside of the steel material by the shell element. The second calculation unit that calculates the second value, which is the maximum principal strain of the outermost layer, and
Using the finite element method using the shell element, a third calculation unit that executes deformation analysis according to the material properties of the steel material and obtains the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
A determination unit that compares the second value with the third value and determines whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
A rupture prediction device characterized by being equipped with.

(7)
In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction device according to (6), wherein a: the relational expression of the plate thickness t b: the relational expression of the plate thickness t.

(8)
In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture predictor according to (7), which is represented by a relational expression of shell element size α, β, γ: ε _{solid and plate thickness t.}

(9)
The solid element used to obtain the first relational expression is characterized in that the element size is finer than that of the shell element used to obtain the second relational expression (6) to (8). ). The breakage predictor according to any one of.

(10)
The fracture prediction device according to any one of (6) to (9), wherein the steel material is a steel plate of a steel grade of 980 MPa class or higher.

(11)
The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. The first step of inputting the limit VDA bending angle of the steel material and calculating the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated in the first step, and enter the outermost bending of the steel material by the shell element. The second step to calculate the second value, which is the maximum principal strain of the surface layer, and
Using the finite element method using the shell element, the third step of performing deformation analysis according to the material properties of the steel material and acquiring the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
The fourth step of comparing the second value with the third value to determine whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
Break prediction program that causes the computer to execute.

(12)
In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction program according to claim 11, wherein a: the relational expression of the plate thickness t b: the relational expression of the plate thickness t.

(13)
In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture prediction program according to (12), which is represented by a relational expression of shell element size α, β, γ: ε _{solid and plate thickness t.}

(14)
The solid element used to obtain the first relational expression is characterized in that the element size is finer than that of the shell element used to obtain the second relational expression (11) to (13). ) The breakage prediction program described in any one of.

(15)
The fracture prediction program according to any one of (11) to (14), wherein the steel material is a steel sheet of a steel grade of 980 MPa class or higher.

(16)
A computer-readable recording medium comprising recording the fracture prediction program according to any one of (11) to (15).

According to the present invention, the maximum principal strain of the outermost surface layer of the bent steel material to be analyzed can be obtained extremely easily, in a short time, and accurately without any test or simulation. It is possible to accurately predict whether or not bending fracture will occur, that is, to accurately predict bending fracture of steel materials, resulting in a significant reduction in the cost required for product development and a significant shortening of the development period. Can contribute.

FIG. 1A is a perspective view showing how the limit VDA bending angle is obtained on a trial basis. FIG. 1B is an enlarged front view showing how the limit VDA bending angle is acquired on a trial basis. FIG. 2 is a diagram showing the obtained limit VDA bending angle (experimental value) and the maximum principal strain of the bending outermost surface layer at the bending outer top (solid element detailed FEM analysis value) for each plate thickness. FIG. 3 is a characteristic diagram showing the relationship between the distance from the bending outer top and the maximum principal strain of the bending outer outermost layer. FIG. 4 is a characteristic diagram showing the relationship between the limit VDA bending angle and the maximum principal strain of the outermost surface layer of bending. FIG. 5 is a schematic diagram showing an FEM model in which an element size mesh is arranged at the bending center of a steel plate. FIG. 6 is a characteristic diagram showing the relationship between the element size (mm) of the shell element and the average value of the maximum principal strain of the outermost surface layer of bending. FIG. 7 is a block diagram showing a fracture prediction device according to the first embodiment. FIG. 8 is a block diagram showing details of a calculation unit which is a component of the fracture prediction device of FIG. 7. FIG. 9 is a flow chart showing a fracture prediction method according to the first embodiment. FIG. 10 is a flow chart showing details of step S1 of the fracture prediction method of FIG. FIG. 11 is a flow chart showing details of steps S11 and S12 of FIG. FIG. 12A is a perspective view showing how a reaction force is applied to the target member by a punch to cause breakage. FIG. 12B is a schematic view showing how a reaction force is applied to the target member by a punch by an experiment to cause fracture. FIG. 12C is a schematic view showing how a reaction force is applied to the target member by a punch according to the present embodiment to cause breakage. FIG. 13 is a characteristic diagram showing the relationship between the stroke and the reaction force when a reaction force is applied to the target member by a punch. FIG. 14 is a block diagram showing a computer function.

(Basic gist of the present invention)
First, in disclosing the embodiments of the present invention, the basic gist of the present invention will be described.

The present inventor conducted a VDA bending test using steel plates of each steel type (material) and each plate thickness, and obtained the limit VDA bending angle of each steel type and each plate thickness. As a conventional knowledge, it is known that bending fracture occurs when the maximum principal strain of the outermost surface layer of bending reaches the limit value peculiar to the material. The present inventor created an FEM model that reproduced the VDA bending test using a detailed mesh of solid elements using the finite element method (FEM) until the limit VDA bending angle obtained from the VDA bending test was reached. The punch was pushed in, and the maximum principal strain of the outermost surface layer of bending at that time was obtained.

Generally, in FEM analysis, a detailed FEM model (solid model) using a solid element and a FEM model (shell model) using a shell element are mainly used. A solid model is modeled using a three-dimensional solid element such as a tetrahedron, a hexahedron, or a pentahedron. Generally, the analysis time is long, but the analysis accuracy is high. The shell model is modeled using shell elements that are two-dimensional surface elements such as triangles and quadrangles, and generally, the analysis target is a member composed of plates that are thinner than the length and width. It is often used in targeted analysis and has the advantage of short analysis time. Therefore, even analysis with a large-scale model, which cannot be analyzed with a solid model, can be performed by using shell elements.

The VDA bending test is one of the methods for evaluating the bendability of a steel sheet, and is a bending test for a metal material according to the test standard (Non-Patent Document 1) of the German Association of the Automotive Industry of VDA.

The limit VDA bending angle is a value experimentally obtained by the VDA bending test corresponding to the tensile strength characteristics and the plate thickness. As shown in FIGS. 1A and 1B, the steel plate 10 is placed on the pair of rolls 21 and 22, and the punch 20 abuts on the central portion (bending center) of the surface of the steel plate 10 between the pair of rolls 21 and 22. Let me. The steel plate 10 is bent while pushing the punch 20 with the load F, and the bending angle of the steel plate 10 at the timing when the reaction force of the punch 20 reaches the maximum value is defined as the limit VDA bending angle θ (FIG. 1B). The maximum principal strain when the limit VDA bending angle θ is reached at the point P on the back surface corresponding to the contact portion of the punch 20 on the front surface of the steel plate 10 is defined as the maximum principal strain of the outermost surface layer of bending at the outer top of the bending. ..

Conventionally, the V-block method has been used as a method for evaluating the bendability of a steel sheet. In the V-block method, a plurality of punches having different bending radii are produced, bending is performed with the punches having each bending radius, and it is determined at which bending radius the punch causes cracking. Therefore, only the data corresponding to the number of punches produced can be obtained, and only fragmentary results can be obtained. Further, in the determination of the crack of the steel sheet, whether or not a minute crack occurs is a determination criterion, so that the determination by the measurer may vary.
On the other hand, in the VDA bending test, since the limit VDA bending angle can be determined by the bending angle at the maximum load, there is an advantage that variation by the measurer does not occur.

For three types of steel sheets (plate thickness: 1.0 mm, 1.4 mm, 2.3 mm) with different plate thicknesses of 1500 MPa class, the obtained limit VDA bending angle (experimental value) and the outermost surface layer of bending at the outer top of bending The maximum principal strain (detailed FEM analysis value using a solid element of 0.1 mm mesh size) is shown in FIG. Further, with respect to the above three types of steel sheets, the relationship between the distance from the bending outer top and the maximum principal strain of the bending outer outermost layer is shown in FIG.

As shown in FIGS. 1 and 2, the limit VDA bending angles obtained in the experiment were 75 °, 65 °, and 55 ° for each steel plate having a thickness of 1.0 mm, 1.4 mm, and 2.3 mm. ..

A FEM model that reproduces the VDA bending experiment is created, and a solid element with high strain reproduction accuracy is used, and a detailed mesh with a size of, for example, about 0.1 mm is used for modeling. Using this FEM model, in the VDA bending experiment, the punch is pushed to the bending angle where the fracture occurred (the limit VDA bending angle), the maximum principal strain on the bending outside of the steel sheet at that time is obtained, and the relationship with the limit VDA bending angle is determined. investigated. As a result, as shown in FIGS. 2 and 3, the maximum principal strain of the outermost surface layer of the bending at the outer top of the bending is 0.386, 0. It was 396,0.393, showing a value that can be regarded as almost the same. That is, it was found that the maximum principal strain on the bent outer side at the upper part of the bent outer side has almost the same value regardless of the plate thickness, even though the limit VDA bending angle changes due to the different plate thickness of the steel plate. rice field. Hereinafter, for convenience of description, the maximum principal strain of the outermost surface layer of the bend at the outermost surface of the bend is simply referred to as "the maximum principal strain of the outermost surface layer of the bend".

When the FEM model consisting of detailed solid elements is used, the present inventor applies bending deformation to the limit VDA bending angle at each plate thickness even if the plate thickness is different if the steel plate is of the same steel type. Using the above findings that the maximum principal strains of the outermost surface layer of bending are almost equal, the limit VDA bending angles at various plate thicknesses were obtained from FEM analysis. Similarly, for other steel types, analysis was performed using an FEM model consisting of detailed solid elements, and the limit VDA bending angle at each plate thickness was obtained. FIG. 4 shows the results of plotting the relationship between a large number of limit VDA bending angles and the maximum principal strain of the outermost surface layer of bending at various steel types and various plate thicknesses thus obtained. In FIG. 4, the plate thickness is shown as t ₁ , t ₂ , and t ₃ .

From FIG. 4, the present inventor has found that the maximum principal strain of the outermost surface layer of bending due to the solid element is expressed as a function of the limit VDA bending angle and the plate thickness. The present inventor scrutinized this finding to embody it, and found that the maximum principal strain of the outermost surface layer of bending due to the lid element is formulated by the linear logarithm of the limit VDA bending angle for each plate thickness regardless of the steel type. I found that it can be transformed.

Bending by a solid element The maximum principal strain of the outermost layer is ε _solid , the limit VDA bending angle is θ, and ε _solid is expressed as follows using coefficients a and b.
ε _solid = a · ln (θ) ＋ b ・ ・ ・ (1)

The coefficient a and the coefficient b are expressed by the relational expression of the plate thickness t. This relational expression is determined to fit the test result by the above equation (1), and the form of the equation is not particularly limited, but can be expressed by, for example, a polynomial of plate thickness t.

Since a large-scale model is required for collision deformation simulation of automobile parts, for example, it takes a huge amount of calculation time to perform FEM analysis using solid elements with detailed element sizes for a full car model, which is not realistic. .. Therefore, in such a case, FEM analysis using a shell element having a coarse element size of about 1 mm to 5 mm is generally performed. When modeled with a shell element, the strain is averaged within the element in a part where the strain is locally concentrated, such as the bending top of a steel plate, so the coarser the element size, the less the strain. And becomes a smaller value. Therefore, there arises a problem that the calculated strain value differs depending on the element size used.

The present inventor created an FEM model that reproduced the VDA bending test with a shell element of 0.7 mm mesh size, which is coarser than the 0.1 mm mesh size used in the detailed FEM analysis using solid elements, and created an FEM model of the outermost surface layer of bending. The maximum principal strain was obtained. Specifically, as shown in FIG. 5, the steel plate 10 is placed on the pair of rolls 21 and 22, and the punch 20 is placed at the center portion (bending center) of the surface of the steel plate 10 between the pair of rolls 21 and 22. The steel sheet 10 is bent in contact with each other, and the maximum principal strain of the outermost surface layer of bending is obtained from the obtained limit VDA bending angle. Since the FEM model created by the shell element has a planar shape with no apparent thickness, the steel plate 10 is slightly tilted from the horizontal plane in FIG. 5 for convenience of illustration. Here, using an FEM model in which the mesh was placed at the bending center of the steel sheet in the shell element, the range of the element for reading the maximum principal strain of the obtained outermost surface layer of the bending was changed and averaged within each reading range. Specifically, the range of elements for reading the maximum principal strain of the outermost surface layer of bending is defined as the bending center of the steel sheet 10 and, for example, 1, 3, 5, 7 elements on the left and right thereof, that is, 0.7 mm, 2.1 mm, 3.5 mm. Assuming 4.9 mm, the reading range, that is, g ₁ , g ₂ , g ₃ , g ₄ in FIG. 5 is replaced with the element size of the shell element, and the maximum principal strain of the outermost surface layer of bending is g ₁ , g ₂ , g _3. , G ₄ was read from the range, and the average value of each was obtained. FIG. 6 shows the relationship between the element size (mm) of the shell element obtained in this way and the maximum principal strain of the outermost surface layer of bending in the shell element.

Similar to FIG. 6, the present inventor performed an analysis using a FEM model composed of shell elements of various steel types and various plate thicknesses, and performed analysis using the element size (mm) of the shell element and the bending outer outermost layer of the shell element. Many relationships with the maximum principal strain were obtained. As a result of investigating the relationship between these data and the data obtained from FIG. 4, the maximum principal strain of the outermost surface layer of bending in the FEM model using the shell element is the element size of the shell element, the plate thickness, and the above-mentioned solid. It was found that it is expressed as a function of the maximum principal strain of the outermost surface layer of bending due to the element. The present inventor scrutinized this finding to embody it, and found that the maximum principal strain of the outermost surface layer of bending in the FEM model using the shell element is an equation using the power of the element size of the shell element regardless of the steel type. We found that it could be formulated.

Bending by the shell element The first value of the maximum principal strain of the outermost layer is ε _shell , the element size of the shell element is S, and the ε _shell is expressed as follows using the coefficients α, β, and γ.
ε _shell = αS ^β + γ ・ ・ ・ (2)

Here, α, β, and γ are _{relational expressions of ε solid} and plate thickness t. This relational expression is determined to fit the analysis result of the FEM model using the shell element by the above equation (4), and the form of the equation is not particularly limited, but for example, from the plate thickness t and ε _solid. Is a linear expression.

_{The maximum principal strain ε solid} of the outermost surface layer of bending represented by the equation (1) was obtained by the present inventor based on the result of FEM analysis using a solid element having a detailed element size. _{The maximum principal strain ε cell} of the outermost surface layer of bending represented by the equation (2) was obtained by the present inventor based on the result of FEM analysis using a shell element having a coarse element size. ε _shell is a value that reflects _{ε solid} obtained by Eq. (1). The ε _shell is a value obtained by using a shell element, but _{is a value specified by ε so lid} obtained by using a detailed solid element, and is a highly accurate value corresponding to the solid element.

In the present invention, the maximum principal strain of the outermost surface layer of bending, which is defined by using the FEM by the solid element, is expressed by the function of the plate thickness and the limit VDA bending angle (1), and the default by using the FEM by the shell element. Eq. (2) is used, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost surface layer of bending due to the solid element. First, the plate thickness and the limit VDA bending angle of the steel material to be predicted to break are input to the equation (1). _{As a result, the maximum principal strain ε solid of the} outermost surface layer of bending due to the solid element is calculated. Subsequently, the element size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the calculated ε _solid are input to the equation (2). _{As a result, the maximum principal strain ε shell of the} outermost surface layer of the bend, which is the fracture criterion of the bend due to the shell element, is calculated.

On the other hand, using a tensile strength characteristics corresponding to the material properties of the steel sheet, subjected to collision deformation simulation by FEM analysis using a shell element, to obtain the maximum principal strain epsilon ^* _{sh ell} bending outside the outermost layer of the steel sheet.

Thus obtained, in comparison maximum during a collision deformation simulation bent outside the outermost layer of the principal strain epsilon ^* _shell, the maximum principal strain epsilon _{s hell} is a cutaway criteria bent outside the outermost layer of the bend, the collision deformation It is determined whether or not fracture occurs at the bending deformation site of the steel plate in.

Specifically, when the epsilon ^* _shell which changes momentarily reached a criteria of fracture epsilon _{shel l,} i.e.,
ε ^* _shell / ε _shell = 1
When becomes, it is determined that the steel plate has broken, and the mesh corresponding to the broken portion is deleted.

_{According to the present invention, ε cell} , which is the maximum principal strain of the outermost surface layer of bending, which is the fracture fracture criterion of the steel material to be analyzed, can be obtained very easily, in a short time, and accurately without using a test or FEM simulation. It is possible to accurately predict the bending fracture in the sudden deformation simulation.

(First Embodiment)
In the first embodiment, the fracture prediction method and the fracture prediction device for the steel sheet will be described in detail with reference to the drawings. FIG. 7 is a block diagram showing a fracture prediction device according to the first embodiment, and FIG. 8 is a block diagram showing details of a calculation unit which is a component of the fracture prediction device of FIG. 7. 9 is a flow chart showing a fracture prediction method according to the first embodiment, FIG. 10 is a flow chart showing details of step S1 of the fracture prediction method of FIG. 9, and FIG. 11 shows details of steps S11 and S12 of FIG. It is a flow diagram.

As shown in FIG. 7, the fracture prediction device according to the present embodiment includes a calculation unit 1, an acquisition unit 2, and a determination unit 3. The calculation unit 1 calculates the maximum principal strain of the outermost surface layer of bending as the fracture limit surface strain at the time of bending deformation of the steel sheet by using the equations (1) and (2), as shown in FIG. , The first calculation unit 11 and the second calculation unit 12 are provided. The acquisition unit 2 acquires the maximum principal strain of the outermost surface layer of the bent steel sheet in the collision deformation simulation. The determination unit 3 compares the value obtained by the calculation unit 1 with the value obtained by the acquisition unit 2 to determine whether or not the steel sheet is broken.

When predicting the breakage of a steel sheet, first, various information is input to the calculation unit 1 from an input file. Various information includes the thickness of the steel plate, the limit VDA bending angle, the element size of the shell element of the model for performing the collision deformation simulation, and the like. The plate thickness and the limit VDA bending angle are input to the first calculation unit 11, and the plate thickness and the element size are input to the second calculation unit 12.

As the steel type, it is particularly preferable to target a steel sheet of 980 MPa class or higher. Since the steel sheet in which bending fracture is a problem is mainly a high-strength material, in the present embodiment, a steel sheet of 980 MPa class or higher is applied to the fracture prediction as a specific index of the high-strength material.

The limit VDA bending angle corresponding to the tensile strength characteristics and the plate thickness can be obtained by the user by experiment when predicting the fracture of the steel sheet, or can be obtained and input from the database created based on the experiment. good.

In step S1, the calculation unit 1 calculates the maximum principal strain of the outermost surface layer of the bent steel sheet at the time of bending deformation of the steel sheet by using the equations (1) and (2). Step S1 includes steps S11 and S12.

First, the first calculation unit 11 uses the input plate thickness and the limit VDA bending angle in step S11 to determine the bending outermost of the steel plate, which is defined in the FEM model by the solid element by the equation (1). Calculate the maximum principal strain ε _solid of the surface layer.

Following step S11, the second calculation unit 12 uses the input plate thickness, element size, and ε- _strain calculated in step S11 in step S12, and uses the FEM model using the shell element according to the equation (2). _{The specified maximum principal strain ε shell} of the outermost surface layer of the bent steel sheet is calculated.

On the other hand, the acquisition unit 2 fits the tensile strength characteristics corresponding to the element size of the shell element and the material characteristics of the steel plate, for example, the stress-strain curve or the stress-strain curve, which are input in step S2, by the Swift equation. FEM analysis using shell elements by collision deformation simulation is performed using the swift coefficient obtained in the above, and the maximum principal strain ε ^* _cell of the outermost surface layer of the bent steel plate is obtained.

Following steps S1 and S2, in step S3, the determination unit 3 has the maximum principal strain ε _shell of the outermost bending surface layer obtained in step S1 and the maximum principal strain ε of the outermost bending outer layer obtained in step S2. ^* _{Compare with shell} to determine whether or not fracture occurs at the bending deformation site of the steel plate in collision deformation.

Specifically, in step S3, when the ε ^* _{shell, which changes from moment to moment, reaches the ε shell} , which is the criterion for fracture, that is,
ε ^* _shell / ε _shell = 1
When the result becomes, the determination unit 3 determines that the steel sheet has broken, and erases the mesh corresponding to the broken portion.

(Example)
The result of verifying the prediction accuracy of the fracture prediction method according to this embodiment will be described. Regarding the relationship between the stroke (mm) and the impactor reaction force (kN) when the target steel material 30 is pressed down by the impactor using the three-point bending jig shown in FIG. 12A in order to simulate the collision input to the automobile member. Examined. As the target steel material 30, a hat member simulating an automobile member made of a 2.0 GPa class hot stamping material having a plate thickness of 1.6 mm was used. FIG. 12B shows the situation where the fracture occurred by the experiment, and FIG. 12C shows the situation where the fracture was predicted by this example. In the case of FEM analysis considering fracture by the fracture prediction method according to this example, the case of conventional FEM analysis not considering fracture was investigated as a comparative example. In the FEM analysis according to this embodiment, NSafe-MAT, which is a program for predicting material breakage, is used, the mesh size is 2 mm, the limit VDA bending angle is 40 °, and the plate thickness is 1. It was input as 6 mm.

The obtained results are shown in FIG. FIG. 13 is a characteristic diagram showing the relationship between the stroke (mm) and the impactor reaction force (kN) when the target member is pressed down by the impactor. In the test, it was observed that the fracture occurred from the bent part. On the other hand, also in this embodiment, there is a part where the element is deleted in anticipation of the breakage, which is almost the same as the breakage part observed in the test. From FIG. 12, it can be seen that, according to the FEM analysis in which the fracture is taken into consideration by the fracture prediction method according to the present embodiment, the state in which the load suddenly decreases due to the occurrence of the fracture is reproduced. On the other hand, in the FEM analysis of the comparative example that does not consider the fracture, which is a conventional technique, it can be seen that the load reduction due to the occurrence of the fracture cannot be reproduced because the fracture cannot be predicted.

As described above, according to the present embodiment, the maximum principal strain of the bent outer surface layer of the steel material to be analyzed can be obtained extremely easily, in a short time, and accurately without using tests or simulations. As a result, it is possible to accurately predict whether or not bending fracture occurs in the projecting deformation portion, that is, to accurately predict bending fracture of the steel material. As a result, it is possible to omit the collision test on the actual automobile member or to significantly reduce the number of collision tests. In addition, since the steel plate that prevents breakage in the event of a collision can be designed on a computer, it will contribute to a significant cost reduction and shortening of the development period.

(Second embodiment)
Calculation unit 1 (first calculation unit 11 and second calculation unit 12 in FIG. 8), acquisition unit 2, and determination unit 3 shown in FIG. 7, which are components of the fracture prediction device according to the first embodiment described above. May be realized by dedicated hardware. In addition, each of the above components is composed of a memory and a CPU (Central Processing Unit), and the functions are realized by loading and executing a program for realizing various functions of each component in the memory. There may be.

In addition, a computer-readable record of a program for realizing the various functions of each of the above components (a program for executing steps S1 (steps S11 to S12 in FIGS. 10 and 11) to S3 in FIG. 9). The processing of each of the above-mentioned components may be executed by recording on a medium, having the computer system read the program recorded on the recording medium, and executing the program. The term "computer system" as used herein includes hardware such as an OS and peripheral devices.

Further, the "computer system" may include a homepage providing environment (or display environment) as long as the WWW system is used.
Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, and a storage device such as a hard disk built in a computer system. Further, a "computer-readable recording medium" is a communication line for transmitting a program via a network such as the Internet or a communication line such as a telephone line, and dynamically holds the program for a short period of time. It may also include a program that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or a client in that case. Further, the above program may be for realizing a part of the above-mentioned functions, and may be further realized for realizing the above-mentioned functions in combination with a program already recorded in the computer system. ..

As a specific example, the breakage prediction device and the breakage prediction method shown in the present embodiment are carried out by the computer function 100 as shown in FIG.
The computer function 100 includes a CPU 101, a ROM 102, and a RAM 103. Further, the controller (CONSC) 105 of the operation unit (CONS) 109 and the display controller (DISPC) 106 of the display (DISP) 110 as a display unit such as a CRT or an LCD are provided. Further, it includes a hard disk (HD) 111, a controller (DCONT) 107 of a storage device (STD) 112 such as a flexible disk, and a network interface card (NIC) 108. The functional units 101, 102, 103, 105, 106, 107, and 108 are configured to be communicably connected to each other via the system bus 104.

The CPU 101 comprehensively controls each component connected to the system bus 104 by executing the software stored in the ROM 102 or the HD 111 or the software supplied from the STD 112. That is, the CPU 101 controls to realize the operation in the present embodiment by reading the processing program (structure design support program) for performing the above-mentioned operation from the ROM 102, HD111, or STD112 and executing the processing program. I do. The RAM 103 functions as a main memory or a work area of the CPU 101.

CONSC105 controls the instruction input from CONS109. DISPC 105 controls the display of DISP 110. The DCONT 107 controls access to HD111 and STD112 that store boot programs, various applications, user files, network management programs, and the above processing programs in this embodiment. NIC 108 exchanges data bidirectionally with other devices on the network 113.
Instead of using a normal computer terminal device, a predetermined computer or the like specialized for the fracture prediction device may be used.

The present invention can be used, for example, in industries related to steel sheets that are public for automobile parts.

Claims (16)

The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. The first step of inputting the limit VDA bending angle of the steel material and calculating the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated in the first step, and enter the outermost bending of the steel material by the shell element. The second step to calculate the second value, which is the maximum principal strain of the surface layer, and
Using the finite element method using the shell element, the third step of performing deformation analysis according to the material properties of the steel material and acquiring the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
The fourth step of comparing the second value with the third value to determine whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
A fracture prediction method characterized by being equipped with. In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction method according to claim 1, wherein a: the relational expression of the plate thickness t b: the relational expression of the plate thickness t. In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture prediction method according to claim 2, wherein the shell element size α, β, γ: ε _solid and the plate thickness t are represented by a relational expression. The solid element used to obtain the first relational expression has an element size finer than that of the shell element used to obtain the second relational expression, claims 1 to 3. The fracture prediction method according to any one item. The fracture prediction method according to any one of claims 1 to 4, wherein the steel material is a steel sheet of a steel grade of 980 MPa class or higher. The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. A first calculation unit that inputs the limit VDA bending angle of the steel material and calculates the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated by the first calculation unit into the equation, and bend the outside of the steel material by the shell element. The second calculation unit that calculates the second value, which is the maximum principal strain of the outermost layer, and
Using the finite element method using the shell element, a third calculation unit that executes deformation analysis according to the material properties of the steel material and obtains the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
A determination unit that compares the second value with the third value and determines whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
A rupture prediction device characterized by being equipped with. In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction device according to claim 6, wherein a: the relational expression of the plate thickness t b: the relational expression of the plate thickness t. In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture prediction apparatus according to claim 7, wherein the size α, β, γ: ε _{solid of the} shell element and the plate thickness t are represented by the relational expression. Claims 6 to 8, wherein the solid element used to obtain the first relational expression has an element size finer than that of the shell element used to obtain the second relational expression. The breakage predictor according to any one item. The fracture prediction device according to any one of claims 6 to 9, wherein the steel material is a steel plate of a steel grade of 980 MPa class or higher. The first relational expression, which expresses the maximum principal strain of the outermost surface layer of bending as a function of the plate thickness and the limit VDA bending angle, which is defined by using the finite element method with solid elements, is the plate thickness of the steel material to be predicted for fracture and the plate thickness. The first step of inputting the limit VDA bending angle of the steel material and calculating the first value which is the maximum principal strain of the outermost surface layer of bending due to the solid element of the steel material.
The second relation, which is defined by using the finite element method with shell elements, expresses the maximum principal strain of the outermost bending layer as a function of the size of the shell element, the plate thickness, and the maximum principal strain of the outermost bending layer due to the solid element. Enter the size of the shell element of the deformation analysis model, which is the target of fracture prediction, the plate thickness of the steel material, and the first value calculated in the first step, and enter the outermost bending of the steel material by the shell element. The second step to calculate the second value, which is the maximum principal strain of the surface layer, and
Using the finite element method using the shell element, the third step of performing deformation analysis according to the material properties of the steel material and acquiring the third value, which is the maximum principal strain of the outermost surface layer of the bending of the steel material, and
The fourth step of comparing the second value with the third value to determine whether or not fracture occurs at the bending deformation portion of the steel material in deformation.
Break prediction program that causes the computer to execute. In the first relational expression, the maximum principal strain of the outermost surface layer of bending due to the solid element is ε _solid .
ε _solid = a · ln (θ) + b
The fracture prediction program according to claim 11, wherein a: the relational expression of the plate thickness t b: the relational expression of the plate thickness t. In the second relational expression, the maximum principal strain of the outermost surface layer of bending due to the shell element is ε- _shell .
ε _shell = αS ^β + γ
S: The fracture prediction program according to claim 12, wherein the shell element size α, β, γ: ε _solid and the plate thickness t are represented by a relational expression. The solid element used to obtain the first relational expression has an element size finer than that of the shell element used to obtain the second relational expression. The breakage prediction program according to any one of the items. The fracture prediction program according to any one of claims 11 to 14, wherein the steel material is a steel sheet of a steel grade of 980 MPa class or higher. A computer-readable recording medium comprising recording the fracture prediction program according to any one of claims 11 to 15.

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